/Engine/Source/Runtime/SlateCore/Public/Application/SlateApplicationBase.h
Associates a top level Slate Window with a native window and ensures that it is tracked properly by the application. Calling this method will cause the window to be displayed (unless specified otherwise), so be sure to associate content with the window object you're passing in first!
a reference to theSWindowthat was just added.
[TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWindow](API\Runtime\SlateCore\Widgets\SWindow) > AddWindow ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWindow](API\Runtime\SlateCore\Widgets\SWindow) > InSlateWindow, const bool bShowImmediately )